The present invention refers to a measurement probe for measuring the mass of a flowing fluid, particularly the intake air of an internal-combustion engine, having at least one temperature-dependent first resistor which is arranged in the path of the fluid and is part of a bridge circuit and the temperature and/or resistance of which is controlled as a function of the flowing mass, the actuating variable being a measure of the mass of the flowing fluid.
In known electrothermal air-mass sensors, a resistance layer which is applied to a flat support is traversed by a current which results in heating. Depending on the speed of flow of the air surrounding the sensor a greater or lesser amount of heat is removed from the resistance layer and the support. By the use of a resistance material having a positive or negative temperature coefficient, the flow can be so controlled that a constant temperature above the temperature of the air is obtained. The current required for this is then a measure of the velocity of flow.
One important field of use of such sensors is the measuring of the mass of air drawn in by internal combustion engines. This measurement is particularly important in order to control the combustion process of the internal-combustion engine so as to minimize the proportion of injurious substances upon the combustion.
Particularly in the case of Otto engines there are operating conditions with pulsating flow or even reverse flow which lead to erroneous measurements in known air-mass meters.
It has furthermore been found that the local heat-transfer coefficient is greatest in the vicinity of that edge of the sensor which is opposite the direction of flow and that it decreases with increasing distance from said edge. In this way, a temperature gradient is produced along the resistance layer and the support in the direction of flow, said gradient being dependent on the velocity of flow and causing a relatively long response time of the known sensors.
In order to reduce the response time, it is already known to dimension the current paths produced in the resistance layer in such a manner that the resultant electric heating output per unit of area is in a constant relationship to the local heat-transfer coefficient.